1. Field of the Invention
The present invention relates to an analog-to-digital conversion circuit, and more particularly to a technique for reducing power consumption in the analog-to-digital conversion circuit.
2. Description of the Related Art
Recently, cellular phones have been provided with various additional functions such as an image shooting function, an image reproducing function, a moving image shooting function, and a moving image reproducing function. Accordingly, there are increasing needs for an analog-to-digital converter (hereinafter referred to as an “AD converter”) incorporated into the cellular phone to operate at lower voltages with lower power consumption. In particular, one-chip LSIs incorporating an AD converter has employed finer design rules year by year, and thus required lower supply voltages. As an AD converter having high conversion accuracy, known to those skilled in the art is a multi-stage pipelined AD converter (for example, see Japanese Patent Laid-Open Publication No. Hei 9-69776, its entirety and FIG. 1).
Reducing the overall voltage range for an analog signal to realize lower power consumption would readily cause degradation in conversion accuracy clue to decreases in S/N ratio. In general, the pipelined AC converter incorporates a sampling circuit with capacitors, and its mean square voltage of thermal noises is expressed by the following equation:{overscore (v2)}∝kT/C  Equation 1
Assuming that the amount of thermal noise is constant as expressed by Equation 1, a reduction in voltage range for an analog signal would cause a corresponding decrease in S/N ratio. For example, an overall accuracy of 10 bits would require an S/N ratio of about 60 dB or more. To compensate the aforementioned S/N ratio when the voltage range is reduced, it is found from Equation 1 that C has to be increased to thereby reduce the thermal noise. For example, halving the voltage range for an analog signal would make it necessary to increase C by four times to compensate for the halving. This causes an increase in size of the capacitor leading to an increase in circuit area. On the other hand, uncompensated noises themselves would cause degradation in characteristics.